Pressure sensitive adhesives derive their name from the way they work. Unlike other types of glues that form a bond with the surface substrate that they are applied to, through evaporating or absorbing a solvent to form a solid material, pressure sensitive adhesives stick to the substrate through the use of slight pressure.
The performance of pressure sensitive adhesives can be characterized through a variety of physical tests. The tests are designed to measure the strength of the adhesive, or its resistance to various environmental conditions. The typical adhesive properties include initial tack, ultimate adhesion, shear resistance, U.V. resistance, solvent resistance, mandrel hold, cold flow, minimum application temperature and service temperature range.
Initial tack is the immediate holding power of the adhesive upon contact with a substrate to which it is applied. An adhesive with high initial tack will grab the substrate quickly. An adhesive with low initial tack will exhibit a low level of adhesion when first applied and may be removed cleanly.
Ultimate adhesion is the ultimate or maximum holding power that the adhesive will achieve as the adhesive penetrates into the substrate. The time required to obtain ultimate adhesion may depend on the stiffness (shear) of the adhesive, the roughness of the substrate and the temperature of the environment. Dwell time can vary from 10 minutes to 24 hours depending on the conditions. Initial tack and ultimate adhesion are not necessarily related. It is possible that an adhesive with high initial tack may not obtain as high an ultimate adhesion as an adhesive with low initial tack.
Shear resistance is a measure of internal cohesive strength of the adhesive. The shear of the adhesive is an indication of how soft an adhesive is. A low shear adhesive (soft) has more of a tendency to flow (resulting in higher initial tack) and has a higher chance that the adhesive will split apart if put under stress. A high shear adhesive (firm) is less likely to split under stress because of its good internal cohesive strength, and will be less likely to flow. The ability of an adhesive to flow, not only affects the initial tack but can determine the amount of "edge ooze". Edge ooze is a term used to describe the flow of adhesive over the edge of the substrate to which it is applied. Edge ooze causes sticky edges and can cause feeding problems and contamination in some printing processes when the adhesive is used in making labels.
U.V. resistance is the ability of the adhesive to resist prolonged exposure to ultraviolet light without loosing adhesion or changing color.
Solvent resistance is the ability of the adhesive to withstand exposure to solvents without loosing adhesion. Solvents could include water, alcohol, petrochemical solvents, organic solvents, plasticizers and the like. The solvents could be present in the environment that the adhesive is used in or could be within the substrate to which the adhesive is applied.
Mandrel hold is the ability to adhere to a curved or tight radius surface. Good mandrel hold is the resistance to "flag" or "wing up".
Cold flow is the ability of the adhesive to flow into the substrate to which it is applied at temperatures below normal. Adhesives with good cold flow are considered "cold temperature" or "all temperature" adhesives.
Minimum application temperature is the lowest temperature at which an adhesive will function. A pressure sensitive adhesive will begin to stiffen and lose its adhesive properties as the temperature decreases. Most adhesives have a minimum application temperature of 40-50.degree. F. The minimum application temperature is effected by what is known as the "glass transition temperature", the temperature where the adhesive crystallizes and becomes solid. Cold temperature adhesives have minimum application temperatures as low as -20.degree. F.
Service temperature range is the temperature range that the adhesive will function after the adhesive has been applied to a substrate and allowed to build to ultimate adhesion. Most adhesives have a range as low as -65.degree. F. and as high as 200.degree. F. with paper substrates and 300.degree. F. with film substrates.
The pressure sensitive adhesives materials of the prior art are monolithic materials and as a result have properties that are limited to such monolithic materials. While these monolithic materials may exhibit adhesives with desirable properties (e.g., initial tack, shear resistance,) for certain applications, there is a limitation as to the various combinations of properties that can be provided by such monolithic adhesives. There is no universal pressure sensitive adhesive having acceptable properties for all applications. It would be advantageous to provide a pressure sensitive adhesive material that offered a wider selection of improved properties and thus applications of use. The present invention provides such advantages.
U.S. Pat. No. 3,627,559 discloses an adhesive tape comprised of a backing, a layer of a water soluble gum, and a discontinuous layer of a pressure-sensitive adhesive on the gum. The reference indicates that when applied to a dry substrate, the tape becomes peelable or releasable on being moistened. When applied to a wet substrate, the tape is resistant to peeling or releasing on being moistened.
U.S. Pat. No. 5,024,880 discloses phase-separated, polymerized, cellular pressure-sensitive adhesive membranes possessing a high degree of compression recovery comprising from about 70 parts to about 98 parts of an acrylic polymer, and from about 30 parts to about 2 parts of a saturated hydrocarbon elastomer or blend of hydrocarbon elastomers containing at least one segment having a lower glass transition temperature than the acrylic copolymer. The reference indicates that the adhesive has at least a first phase and a second phase, the first phase consisting primarily of the elastomer, and at least one phase is a continuous phase. The reference also indicates that the disclosed cellular pressure-sensitive adhesive membranes have improved low temperature shock resistance over otherwise identical cellular pressure-sensitive adhesive membranes containing no elastomer.